What is an RSIN number?

RSIN, KVK number and VAT number side by side

Dutch company data uses three numbers and they get confused constantly, especially by teams who are used to a single company identifier. They do three different jobs.

The practical split: to check whether a company exists, use the KVK number. When you work with tax filings or annual accounts, you meet the RSIN. When you issue an invoice, you need the VAT number.

Where the RSIN comes from

The RSIN is assigned when a legal entity is first registered. The Chamber of Commerce issues it and the tax authority uses it. That shared ownership is why it appears to have two names: in registry context it is the RSIN, in tax context you meet the same nine digits as the numeric part of the VAT identification number.

That numeric part is where most integrations go wrong. The VAT number of a BV is NL + the RSIN + B01. It is not NL + the KVK number + B01. The RSIN is nine digits and the KVK number is eight, so pasting the KVK number into that pattern produces a string that looks right and that the tax authority never issued.

The B01 suffix is a subnumber. It is usually B01, but a business with more than one VAT-liable unit can have B02 or higher. That is the second reason the pattern is a starting point rather than an answer, and why validating against VIES is the only way to know a number exists.

When you actually need it

• Annual accounts. Filed accounts at the Chamber of Commerce are keyed on RSIN, not on KVK number.
• Tax correspondence. The Dutch tax authority addresses legal entities by RSIN.
• UBO and KYC work. When mapping a group structure, the RSIN is often the more stable key, because it does not move when a trade name changes.
• Joining public datasets. Many Dutch government and subsidy datasets publish on RSIN.

What the RSIN does not tell you: whether a business is currently trading, and whether a VAT number is valid. Neither is derivable from it.

Going from an RSIN to a company

If you have an RSIN from a tax or government file and need the company behind it, search on it directly. You get the KVK number back, plus the name, the city and whether the registration is still active.

curl "https://api.kvkbase.nl/v1/lookup/rsin/850123458" \
  -H "Authorization: Bearer YOUR_API_KEY"

With that KVK number you then fetch the full profile, which returns the statutory name, the legal form, the address, the branch count and the derived VAT number with a live VIES check on it.

curl "https://api.kvkbase.nl/v1/lookup/68750110?enrich=true" \
  -H "Authorization: Bearer YOUR_API_KEY"

Note that the RSIN itself is not in the response. It is a way in, not a field: store it yourself alongside the KVK number if you need it later.

A modelling mistake worth avoiding

Keying your own companies table on RSIN is tempting, because it lines up neatly with tax data. Do not do it if you also store sole traders and partnerships, because they have no RSIN and you end up with a primary key that is empty for a large share of your rows.

Key on the KVK number and keep the RSIN as an optional column. The KVK number is the only identifier every registration has.